Wordpress + PHP: Query posts to find those that contain a custom field array which contains a specific key

Question

I have a two post types: artists and events.

I have associated the artists and events by putting a check list of all artists in the event editor, so that multiple artists can be associated with multiple events. The artist custom field on the event editor is saved as an array of artist post IDs.

On the events page I show the list of artists for each event with the following:

    //Get Event Artists
    $postID = $post->ID;
    $meta = get_post_meta($postID, $_artistMeta->$postID, TRUE);
    $artists = unserialize($meta['_artistMeta'][0]);

    $output = array();
    foreach ($artists['artistName'] as $artist) {
        $theArtist = get_post($artist);
        $output[] = '<li><a href="'.get_permalink($theArtist->ID).'">'.$theArtist->post_title.'</a></li>';
    }

<?php if($output) { ?>
                                <div class="tw-event-artists bg1">
                                    <ul class="nav-inl group me c2 comma-list">
                                        <li class="bo"><?php echo $featured; ?>:</li>
                                        <?php echo implode('', $output); ?>
                                    </ul>
                                </div>
<?php } ?>

The problem I'm trying to find now is to try and "go in reverse". So in my artists archive, I want to find the latest event for each artist.

I need to create a custom query to find event posts whose artists array contains the current artist slug.

I think http://css-tricks.com/snippets/wordpress/custom-loop-based-on-custom-fields/ that tutorial is a good starting place but my PHP chops are simply not good enough to hack it.

How can write this query to get what I want?

Thanks!

My current method of achieving this is like so, placed inside the loop on my artists archive template, but I imagine this will be extremely slow and inefficient because it has to get every single event for every artist.

<?php 
    $artistID = $post->ID;

    $args = array(
        'eventDisplay'=> 'upcoming',
        'posts_per_page' => -1
    );  
    $events = tribe_get_events($args);

    $has_date = false;

    if ($events):
        global $post;
        foreach ($events as $post):
            setup_postdata($post);
            //Get Event Artists
            $postID = $post->ID;
            $meta = get_post_meta($postID, $_artistMeta->$postID, TRUE);
            $artists = unserialize($meta['_artistMeta'][0]);

            foreach ($artists['artistName'] as $artist) {
                if($artist == $artistID) {$has_date = true;}    
            }
        endforeach;
    endif;
?>

<?php if($has_date) {echo 'HAS DATE';} ?>

Note to mods: this question would be a better fit for WPSE. — Chip Bennett
– Chip Bennett, Commented Apr 18, 2012 at 16:08

gradyetc · Accepted Answer · 2012-04-18 02:20:00Z

A few thoughts...

It's not clear based on your examples whether or not you're using the WP_Query class to retrieve your event posts. The arguments passed to tribe_get_events() function suggest that you're just using get_posts() to retrieve event posts.

I would recommend a secondary loop using WP_Query. This allows you to easily query for posts by meta_key/value:

$query_args = array( 'post_type' => 'event', 'meta_key' => '_artist_id', 'meta_value' => '5' );

$event_query = new WP_Query( $query_args );

if( $event_query->have_posts() ) {
    while( $event_query->have_posts() ) {
        $event_query->the_post();
        ...
        // standard loop stuff goes here

This would return only event posts that had an _artist_id value of 5.

See this page for details: http://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters

The caveat is that you can't use that with post meta values that are serialized. If you wanted to query by meta value, you would need to modify how you're storing event/artist data to break out fields you want to query by in to their own single post meta fields.

However, a nice bonus for using WP_Query is that when posts are retrieved from the database all of their associated post meta is fetched as well and cached in memory. This means that calling get_post_meta() from inside a WP_Query loop will not generate additional database queries, as those values are stored in memory and get_post_meta() is wise enough to always check there first.

A bigger issue your question hints at is one that many consider to be a shortcoming with WordPress -- it doesn't provide a bulletproof built-in standardized mechanism for relating posts to posts.

For the gory details, check out the comment thread on this trac ticket: http://core.trac.wordpress.org/ticket/14513

A popular plugin that gets around the issue by creating a new database table to hold many-to-many relationships: http://wordpress.org/extend/plugins/posts-to-posts/

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